26 DHT Pre-amplifier (Gen3)

 An updated DHT preamp

It’s been a while since I played with preamps. Here is my third iteration of the 26 preamp (please refer to previous version Gen 1 and Gen 2). I love the sound of this valve as many of you out there. I found my previous build a bit inflexible to modify bias points and play around so decided to go back to the workshop and update my preamp, again. The design is very simple as you can see in the figure below. I’m reusing an existing supply but you can get away with a 200V HT and probably a 15V raw DC for the filament circuit. Key characteristics are:

  1. Filament bias as I’m a fan of it, no output or input caps sir!
  2. Added a Salas SSHV2 regulator to provide HT voltage flexibility and vary the operating point of the 26. The SSHV2 is extremely quiet so will give a clean HT supply to the 26. Yes, the lovely colour of the glow valves are lost in this version 🙁
  3. The Kelvin capacitor C1 (ala Morgan Jones) will help keeping Salas noise input low, specially HF.
  4. R1 is a simple resistor to measure the anode current. It is located in a handy place to allow easy measurements
  5. T1 is the Lundahl LL2745 in Alt R mode for 5.6:1 ratio. This provides the lowest gain and the lowest output impedance. In my new system, this preamp is more a line stage as don’t need further gain.
  6. SW1 is helpful in case ground loops are to be broken
  7. The Rod Coleman regulators are set to 800mA to starve the DHT and reduce microphonics and distortion. I found 800mA to be better sounding than my previous 760mA.
  8. P1 is my stepped attenuator and R2 will help providing grid bias when breaking before making action of the attenuator.
  9. RF is 2 pairs of 10Ω/20W parallel/series wirewound.

26 Preamp Gen3

The preamp is extremely quiet. The below measurement is with a noisy Sylvania ST valve which picks up significant 50Hz hum. Notice that 100Hz harmonic is very very low (can’t be seen at -100dB). Distortion at 5Vpp input (8Vpp output) is lower than 0.03% which is what you would expect from a 26. With a good selection of valves you can get this down to 0.02% for sure:

26 preamp gen3 THD test1

The most important part of the build work is grounding and avoiding ground loops. A combination of star grounding is recommended. I do the following star ground combination:

  1. Input stage
  2. Filament bias return
  3. Output stage

You definitely want to avoid the filament current ground introducing hum in the output stage so you want to keep the three star ground points separate.

26 DHT Gen3 ready to be tested
26 DHT Gen3 ready to be tested

Listening to the 26 is a fantastic experience. If you haven’t built a DHT preamplifier then do it. The detail and colour of this stage is unique. I found ST valves being more detailed on the top end, a richer treble compared to the Globe ones. Globe ones are sweeter and mellow. Definitely prefer globes for classical and ST for rock and Jazz.

26 Preamp connected to the 4-65a SE Amplifier
26 Preamp connected to the 4-65a SE Amplifier

The preamp is extremely quiet. Best build so far, I can’t hear hum on my high-efficiency FE167E full-range speakers and that is a real challenge.

Frequency Response (updated Feb 2014)

Here is the frequency response of the preamp. There is no Zobel compensation so you can see the HF response to present a dip at very high frequency. Either way, it’s flat overall with a lovely response from 10Hz to 30KHz (-3dB points)

26 preamp gen3 freq response

 

Ale

41 Replies to “26 DHT Pre-amplifier (Gen3)”

  1. Hi Ale
    Just a comment on my findings regarding the rolling of rectifier valves/tubes in the Power Supply, now I have done a few mods in this particular area, one of them was to add a second valve socket & wire it for use as alternative test bed for trying out actual AZ1 types & comparing them to the AZ31 type that I was using in the other, this proved to be vary interesting, as it was found in a very short time that the Mullard AZ31 is far better (in my Pre-amp/system) in every area than the 2 Mesh Plate Telefunken’s (they both sounded the same) that I had acquired recently & not by a small margin, even my wife commented on it. I have yet to try out the solid plate version of the Telefunken, but I will let you know my findings as soon as I do, as I have one on the way, the Mullards were tighter, they were dynamically more explosive, detail & top end extention were better, cymbals less splashy, bells ring forever, there is greater authority all round, solo instruments sit in the middle of the room! Much more detail as well & all this extra from a valve that costs far less!! I also agree with your comments regarding the ST version of the 26’s & therefore prefer them generally with all types of music.

    On a different aspect of the subject regarding your Gen3 version of this Pre-amp, how are you finding it compared to the older version regarding sound quality/noise?

    kind regards
    Barry.

    1. Hi Barry,
      Interesting feedback. Haven’t tried the AZ31 so can’t comment. Are you using the SSHV regulator?

      I do like my newer preamp. Is definitely dead quiet as have a proper grounding scheme. The 5.6:1 step down provides a lower output impedance and running 26 with a tad more current is better in my opinion.

      I’m using ST Sylvania ones, these are my preferred ones.

  2. Hi Ale,
    I am using 5.6:1 step down & a single SSHV regulator for both channels, I am currently running my ST, RCA 26’s at 850mA, as the bass is definalely at it’s tightest, if I run them higher then you start to get a little hardness, any lower than 800mA then the bass becomes soft & recessed, so 850mA is a good compromise. Earlier findings were due to other components & break in times coming into the equation. I also changed the Russian 0.1uF PIO’s (Glow Tube’s) for a pair of Jupiter HT’s, these made a certain difference to staging & Timbre of instruments, I like these they are very natural sounding, I like their copper wire as well, the copper in cotton one is not too expensive.

    Best Barry.

    1. It wouldn’t take too much effort to convert, or to even switch out the circuit in my version, to see how much different the two versions sound, as you have literally only removed the CCS & Glow tube circuit, the only difference there is that I’m using K&K Audio’s IXY Boards(they are bloody good BTW), forget about test measurements, the real test can only be measured by your Ears, if I had time I would do this with all the Valves I have, because in my Heart of Hearts I know they will all require different Filament Currents & for that matter Plate voltages, I see on you later version, you have a slightly higher Plate Voltage, this could also make a big difference. I am going to have to have stab your simpler version, especially if it’s better!!

  3. Hi Ale,

    I have now modified my pre-amp to switch the circuit from Gen 2 to Gen 3, gen 3 is light years better, more depth, faster dynamics less recessed top end & cleaner bass, Keep it simple is the answer in nearly all cases & this is no exception.

  4. Hi Ale,

    I am driving an Audio Research VT100 mk 2 with it, the single ended input is being driven by a Sutherland PHD Phono Stage, a zyx airy cartridge with only 0.24mV output is my main cartridge, I have more than enough gain for my Amphion Xenon Speakers.

    1. What is the input impedance of the Audio Research?
      I’m driving a step up transformer and the low impedance of the gen3 pre is really good. no loss of treble and I’m very happy with the sound so far.

  5. I am using 2 Arms on my Turntable, one is a Moerch DP6 the other is an Arm that you are familiar with, the Audiomods with adjustable VTA on the fly, it’s a very good arm but just falls short of the Moerch, but not by far, the Audiomods seems to be suited to Cartridges that are very compliant, like the VDH’s. I have an old Transfiguration Temper V that I use on it, the Audiomods is astonishing value & beautiful to look at IMHO!

  6. The VT100 input impedance is 100k ohms single ended or 200k ohm balanced. The DL103 with Zu Body is very good if you get it set up well, Jeff Spall can give good advice on the DL 103, it needs a good step up though!!

  7. If you like vintage sound that is really nice then the Ortofon SPU is superb, infact it’s good whatever type of sound you like

  8. Hi Ale,
    On this latest version of your 26DHT P/amp, I can’t seem to achieve 6mA across the 10R resistor for a voltage of 156V, the current here is 5.3mA, I have the shunt regulator set at 45mA(20mA over the 25mA to give optimum regulation, although altering this makes little difference. I am still using the original B+ Raw Supply & I have 212V coming from that supply into the 10uF Kelvin Cap. I can get 6mA, but the plate voltage is then over 170V, do you think that might be a little high?
    Obviously now there is no CCS in circuit to control current accurately.

    I could use the 250V tap on the Weiss transformer to achieve a similar voltage to your 260V.

    kindest regards
    Barry.

    1. Hi Barry, I’d suspect on the state of the 26s. You can run it at 170V but maximum operating voltage is 180V. Look at the specs here. The beauty of using the SSHV2 is that you can dial up or down the output voltage to suit your operating point. I’d be inclined to say that the 26s are not in great condition. Are these new? What have you changed?

  9. Hi Ale,
    Unfortunately, what I thought were some good valves, were not, I tested them on my Sylvania tester, & they seemed fine, but I have since found a pair of Silvertone ST’s that both draw 6.7mA(lucky here I guess to find two so nicely matched), I did have one from the same batch that was as low as 3.8mA(using preamp to test), this was of little use really, I have one Sylvania 26, I will look for some more of them now. Getting good valves poses a bit of problem now especially Sylvania!!

    BTW my VT100 is 100k ohm input impedance, I would have this to be fine.

    kindest regards
    Barry

    1. Indeed. Getting hold of good 26s is not longer easy. I ended up selecting only 4 matched pairs from a batch of about 16 valves. From the three then I tested them on my preamp and only got two which where really quiet. I gave them to a friend and I’m not having a big stock myself!

      100K input impedance is fine. With a 5.6:1 and 300ohms output impedance the 26 preamp can drive that load very well including the cable capacitance. I have a step up input transformer in my amp which makes it harder to drive, but don’t have any loss of treble whatsoever
      cheers, Ale

  10. Hi Ale,
    I have most everything sorted now except for a small hum in the left channel which is enhanced when I touch anything else in the system, I think this must be a grounding issue, I will have a go at installing a buzz bar system & separating everything out properly, otherwise the sound is excellent including the treble. I would love to make this with a copper chassis, but that would make it very expensive.

    thanks
    Barry.

    1. Hi Barry,
      I’d suspect on the left channel valve. did you try different (and reliable) valves? Some defective 26s pick up hum a lot so people tend to shield them with some tin foil grounded to make them quiet. You can do this test by simple touching earth with one hand and putting your other hand close to the valve glass. You will notice that then the hum is gone. You are acting as a screen for electrostatic hum induction on the valve. I prefer discarding these valves and only using ones that are in excellent condition.
      I wouldn’t touch the ground scheme. If is not working you are likely to get hum across both channels. If this is the first time you are having hum in the system, I’d revisit all recent changes to find the root cause of the hum. Hum is very tricky to trace and you may easily end up rewiring all the preamp and finally found that nothing has changed. This can be extremely frustrating, so I’d look at recent changes. Hum may be due to a defective 26, ground loop or induction from other sources like transformers, etc.
      Cheers, Ale

  11. Hi Ale,

    On your this latest version did you ground the secondary’s of LL2745’s? I did have them grounded on the earlier version!!

    Kind regards
    Barry/

  12. hi dear.
    nice to meet you all.
    I try to build a DHT tube preamp with BA postal tube, I’ve used Rod Filament supply, my speakers are 95 dB. and I got a hum issued.

    could anyone help and guide me to make hum free DHT preamp.

    Thanks

    James

  13. Hello Ale,

    Gathering up parts to start assembling this preamp. Would it be possible to get a little bit of gain (I don’t need much, but a little would be nice) by connecting the interstage transformer for less of a step-down without sacrificing too much? (Alt Q? Alt P?)

    Also, how would this work with a TVC? Have you tried an autoformer volume with this circuit?

    1. Hi John, yes you can but the more gain the higher the output impedance will be. This is not ideal if you are planning to drive your TVC. I’d rather look at higher gain elsewhere in your system. You can try and rewire the OT if you get some loss of treble.
      I don’t have an TVC so can’t comment here.
      Thanks
      Ale

      1. One more question: I have a pair of ll1660 from a previous project. Would these do a fair job or should I get a pair of ll2745?

        Thanks again!

        Best,
        John

    1. Hi Val,I never tried this, but it looks like you could get up to 1W @ 0.6% THD when biased at 250V/30mA Vgk=-18V for an 600 ohm load if using a LL1660 4.5:1. Input drive voltage has to be about 36Vpp so will need a driver stage before this 4P1L. For a 300 ohm load you can get 0.6W @ 1% (20Vpp input) or 1W @ 1.46% with 30Vpp input drive.
      What is the target output power your are looking at?
      Thanks
      Ale

      1. The HD650 is spec’ed at .5W, but it could go with as little as 150mW I think. I was hoping this could be my spud headphone amp, but I guess I could do 2 stages if needed.

        Thanks
        Val

  14. Hi Ale,

    Chapeau! for your great work!

    what is your actual HV & Filament RAW schematic for the GEN3?
    Perhaps you has also some parts recommendation… 😉

    Best,
    Oliver

    1. Hi Oliver,
      Thank you. The HV supply diagram is in a blog post. Use the search function. Filament supply is choke input (LL2733 400mH/1.7A) into 22mF/35V Panasonic EPCOS. Then CMRR choke EPCOS 15mH/1.9A into 2x22mF 35V EPCOS caps. Just ask Rod Coleman for the filament board user guide which has tips and guidance to design your own filament raw supply based on your transformer. I have a custom made split bobbin transformer from JMS in the UK. It has multiple taps in the secondary 21V/23V to fit different preamp designs.
      Cheers
      Ale

  15. First I must say what a great design and a great web page you have.

    I build some time ago the 26 gen 3 pre amp. I sounds really great in my system with 300B OTL.

    I was only concerned with the filament current you designed. In my experience when the filament voltage is low it could create filament poisoning, which could result in emission loss. I am using currently a super silvertone with black plates. NOS made in 1942.

    Hence I changed the cathode resistor to 8 Ohms and adjusted the current source to close to 1 amp.

    Amplifier sounds still great, even have the feeling that the highs are slightly more clearer including deeper bass. Going back and forward gave a 85% correct get with topology was used. It also seems that there is more micro detail.

    Would like to learn if others have similar experiences.

    I also use a phase switch on the secondary part of the output transformer to correct the phase. Have lots of old record here the phase is reversed and with some it is audible.

    I am not using the Salas SSHV2 regulator but have 2 OA3 taking care of the HV. (build this amp mostly from scrap box parts.)

    Then I know you do not like it but I used the LM317. I connected a 0.22 microF orange drop capacitor though between tube connector 1 and the ground.

    Thanks for putting me on the track of DHT pre amps. Having a couple of 300B in the box thinking of experimenting with them.

    Cheers

    Robbert

    1. Hi Robbert,
      Glad to hear you like the DHT build. I never experienced any issues with the filament starvation. If you increase filament current will therefore increase the emission and you will get the 26 to provide a lower anode impedance and drive capability. That will generally result in better treble given amp’s input capacitance. The 26 doesn’t have the stones to drive many loads.
      I’m now using a 4P1L, my 26 is dismantled and second 26 preamp was sold. I have a 01a being built at the moment…
      300B rocks, if properly used!
      Super Silvertone are one of my favourites ones. Nice to have the phase inverter switch though.
      Are you using the LM317 to run the filaments? If so, I strongly suggest you try the Rod Coleman filament boards, it will take your preamp to the next level of sound!
      cheers
      Ale

      1. Hi Ale:

        Looking at your thread about the 01 tube. happy soldering.

        Made some changes to the heater supply, no I did not put a Rod Coleman yet in. I made a common noise filter before the LM317. This enhanced the micro detail. Also made ferrite bead in the HT power supply going into the LL2745. Strangely the last reduced the the white (?) noise on the output some what. Taking it out white noise back. Have one turn over the bead. Checking for oscillation, none found even not in the MHz area. Still need to puzzle this one out.

        Have a great holiday season.

        Cheers

        Robbert

  16. Hi Ale,
    There are two options for the LL2745 the 8mA and the 18mA versions. Would there be a advantage to using the 18mA version instead?

    Obviously voltage drop across the transformer will be lower and only 6mA will go across but I fell like the extra room will allow for lower thd.

    Thoughts?

  17. Do you use a dedicated Trans/Raw supply for each 26 filament or shared until the Rod regulator? Any benefit to having each tube on its own dedicated Trans/supply?

    Same Q for the B+ any benefit to two Trans/rectifiers/dedicate chokes, ect? Or is that over the top with no sound improvements?

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